1. Field of the Invention
The present invention relates to a method of and an apparatus for controlling the drive of a parts feeder to feed a number of parts in succession along a part feed track in a parts feed unit by exerting vibration to the parts feed unit using electromagnetic vibration or piezoelectric vibration. More particularly, the invention is concerned with control method and apparatus for a self-excited vibration parts feeder, which is able to drive a parts feed unit in a controlled manner at a resonance frequency without the necessity of providing a particular detecting device mounted on the parts feed unit.
2. Description of the Prior Art
A typical example of parts feeder of the type using electromagnetic vibration includes a bowl having a spiral parts feed track. Briefly stated, the spiral parts feed track is provided on the inside surface of a side wall of the bowl, and the bowl is provided with an attractable member attached to the underside of a bottom wall of the bowl. The attractable member is supported on upper ends of a plurality of spring members, the lower ends of the spring members being secured to a base, with a predetermined angle formed between each of the spring members and the base. The base supports on its upper surface an electromagnet which operates to intermittently pull or attract the attractable member, thereby exerting vibration to the bowl. The base is fixedly mounted on a floor surface via a suitable support member having a cushioning or damping action, such as rubber.
On the other hand, a typical example of parts feeder of the type using piezoelectric vibration includes a bowl having the same construction as the bowl described above, and a plurality of pairs of spring members and piezoelectric elements directly connected together. The pairs of spring members and piezoelectric elements are attached at their upper end lower ends to the bowl and a base, respectively, with an angle formed between each pair of spring member and piezoelectric element and the base.
The above-mentioned parts feeders are driven when the electromagnet or the piezoelectric elements are caused to vibrate by an electric power having a constant frequency (driving frequency). To improve the driving efficiency, a mechanical system is so designed as to have a natural frequency which is resonant with the driving frequency. However, in the parts feeder the natural frequency and amplitude of vibration of the mechanical system are greatly influenced by various factors, such as the bowl, resilient legs, base and support member. In addition, a body of the parts feeder has many uncertain factors. Accordingly, the natural frequency of the parts feeder is difficult to identify. Furthermore, the natural frequency is variable with a change in the weight of parts to be fed and also with a change in the ambient temperature. Under these circumstances, the natural frequency gradually deviates from the driving frequency as the time goes on. It is, therefore, almost impossible to keep the parts feeder driving in a resonating state with a stable vibration amplitude.
The change in the natural frequency and amplitude of vibration of the mechanical system brings about a disturbance in the conveyance of the parts and sometimes slows or interrupts the supply of the parts. Such an interruption of the supply of the parts lowers the amount of parts supplied from the parts feeder, increases a downtime of the parts feeder, and considerably lowers the operation rate of the parts feeder. To avoid this, the operator must endeavor to keep smooth supply of the parts by repeating a tedious adjustment of the parts feeder many times a day. On the other hand, fluctuation of a source voltage is not negligible and exerts a delicate influence on the vibration of the bowl.
In order to overcome the foregoing difficulties, there has been proposed a parts feeder controller such as disclosed in Japanese Patent Laid-Open Publication No. Sho 57-27808. The disclosed parts feeder controller seeks to control the drive of the parts feeder such that the vibration amplitude of an electromagnet is kept always constant to feed the parts in a stably manner regardless of fluctuation of the source voltage or a change in the amount of parts held in the bowl. To this end, the parts feeder controller includes a photoelectric transducer composed of a light projector and a light-sensitive detector arranged in alignment with each other so that light emitted from the light projector toward the light-sensitive detector passes through an air gap between a fixed iron-core and a movable iron-core of the electromagnet. The photoelectric transducer converts a change in the quantity of light passing through the air gap into a current value to detect an amplitude of vibration of the electromagnet. A signal current representing the detected vibration amplitude is fed back to a vibration amplitude control circuit to regulate by phase control the magnitude of current flowing through the electromagnet so that the vibration amplitude of the electromagnet is kept always constant.
Another parts feeder controller disclosed in Japanese Patent Publication No. Sho 52-40118, for example, includes a vibrating condition detector attached to a resilient leg supporting a parts feeder unit. The detector electrically detects the amount of deformation of the resilient leg caused by vibration of a bowl. A signal from the detector is fed back to a power amplifier of a drive unit to cause self-excited oscillation so that the driving frequency is identical to a natural frequency of a mechanical system. The signal from the detector is also used to vary the pulse duration of a current flowing through a drive coil for controlling a driving force in order to keep the amplitude of vibration constant.
The above-mentioned parts feeder controllers each have a detector mounted on the body of the parts feeder exclusively for a purpose of detecting the vibrating condition. The detector thus arranged is not free from damage, may be affected by ambient conditions according to the type used, and generally requires high mounting accuracy.
To eliminate these adverse effects, and regarding resonant frequencies, in particular, it has been conventional practice to preset a natural frequency of a mechanical system when the parts feeder is manufactured. However, there is not much point in presetting the natural frequency because the natural frequency is readily influenced by various factors stated above and also variable as the time goes on.